1. Field of the Invention
The invention generally relates to image processing and, more specifically, to techniques for registering a set of images taken of a scene using different exposure settings to a reference image.
2. Description of the Related Art
Many digital photography applications require a user to capture multiple images of the same scene (i.e., an image stack) using different camera settings. One common application that requires the capture of multiple images is high-dynamic range imaging (HDR). In HDR, multiple images are captured of the same scene using different camera settings for each image. For example, images can be captured using a range of exposure settings. In any single image captured at a particular exposure level, details may be lost in the bright or dark areas of the image. HDR captures these details by stitching or performing a weighted average of the different areas of the scene captured in different images taken using different exposure levels. For example, a low exposure setting may capture more detail in bright areas of the scene while a high exposure setting may capture more detail in dark areas of the scene.
However, one problem with capturing multiple images of the same scene is that the images are typically captured at different times. Because most scenes are dynamic, objects captured at a location in one image may be at a different location in the next image. If the objects in the scene have a large amount of relative motion between images then the algorithms for generating the HDR images may have difficulty determining which pixels in one image correspond to pixels in a different image. Therefore, these image applications usually have the best results when applied to static scenes and where the position of the camera is static between each of the pictures.
In today's world, most people carry and capture images using handheld cameras. Images can be captured using expensive digital single-lens reflex (SLR) cameras or image sensors built into the body of a cellular phone. In other words, most images are captured using hand held devices, which users do not take the time to immobilize with a tripod or other device. Consequently, not only are the objects in the scene dynamic and moving relative to other objects in the scene, but the position of the scene is dynamic relative to the position of the camera.
Conventional HDR algorithms that generate an image from multiple images of a dynamic scene may cause blending artifacts to appear in the generated image. For example, ghosted objects could appear in multiple locations in the blended HDR image. In another example, the blended HDR image could have a blurry appearance overall resulting from camera motion that occurs between capture of consecutive images. Such ghosted object and blurry images result in a blended HDR image with reduced image quality.
Accordingly, what is needed in the art is a more effective approach for registering and modifying the images of an image stack.